The present disclosure relates generally to immersion lithography, such as is used in the manufacture of semiconductor integrated circuits.
Lithography is a mechanism by which a pattern on a mask is projected onto a substrate such as a semiconductor wafer. In areas such as semiconductor photolithography, it has become necessary to create images on the semiconductor wafer which incorporate minimum feature sizes under a resolution limit or critical dimension (CD). Currently, CDs are reaching 65 nanometers and less.
Immersion lithography is a new advance in photolithography, in which the exposure procedure is performed with a liquid filling the space between the surface of the wafer and the lens. Using immersion photolithography, higher numerical apertures can be built than when using lenses in air, resulting in improved resolution. Further, immersion provides enhanced depth-of-focus (DOF) for printing ever smaller features.
The immersion exposure step may use de-ionized water or another suitable immersion exposure fluid in the space between the wafer and the lens. Though the exposure time is short, the combination of the fluid and the photosensitive layer (such as photoresist, or simply “resist”) can cause heretofore unforeseen problems. For example, resist residue comes into contact with the immersion exposure fluid and/or the lens, thereby contaminating the fluid and/or lens and causing defects on the wafer.
It is desired to provide an immersion lithography process that has reduced contamination of the immersion exposure fluid and lens, and reduced defects on the wafer.